The invention relates to an AD-converter, comprising: a plurality of amplifying circuits having generally uniform amplifying and impedance characteristics, each circuit comprising: an input voltage terminal; a reference voltage terminal; and an output voltage terminal; the AD-converter further comprising an input voltage network applying an analog input voltage to each input voltage terminal of said amplifying circuits; a reference voltage network applying reference voltages of a generally uniformly spacing to each reference voltage terminal of said amplifying circuits, wherein a relative functional position of each amplifying circuit is determined by the magnitude of a received reference voltage; an averaging network, comprising of generally identical averaging impedances coupling output voltage terminals of adjacent amplifying circuits; an output voltage network for outputting amplified voltage differences from each of said plurality of amplifying circuits; and a termination network comprising a termination circuit and a termination averaging impedance, said termination circuit receiving an input voltage and a termination reference voltage and comprising an output voltage terminal that is coupled by said termination averaging impedance to an output voltage terminal of an outer amplifying circuit of said array.
Such an AD-converter is known from xe2x80x9cA 6b 500 MSample/s ADC for a Hard Disk Drive Read Channelxe2x80x9d, Tamba, Y, ISSCC""99, pp. 324-325, and comprises an averaging network to reduce the offsets of the amplifiers. However, applying averaging will create a load which depends on the position of the amplifier in the array. Through the averaging network, a shift in the zero crossing is introduced, compared to an ideal network wherein the amplifying circuits do not suffer from offset errors. Due to the finiteness of the array of amplifying circuits, the zero crossing is non-linear towards the outer circuits of the AD-converter array. In order to resemble an infinite array, the disclosed termination network comprises a series of xe2x80x9cdummyxe2x80x9d amplifiers. By using only a series of innermost amplifiers boundary effects are suppressed.
In this known solution, it is clear, that a termination will be more effective if more and more dummy amplifiers are used on either sides of the array, because an increase in the number of amplifiers will resemble an infinite array better. As an example, a number of 6 overrange amplifiers is used in case of an AD-convertor array comprising 16 AD-amplifiers, and for instance, 18 overrange amplifiers are used in case of 63 amplifier array.
The above mentioned disclosed embodiment suffers from the drawback, that linearity is not restored completely, but only approached to a limit, wherein for a better linearity, more and more dummy amplifiers are inserted. Besides an unsatisfying linearity, also the power dissipation and input capacitance of this embodiment are quite unprofitable.
The invention aims to obviate the above mentioned problem and has as one of its goals to provide an improved AD-converter, having improved linearity properties and less power dissipation.
In accordance with the invention, the AD-converter of the above mentioned type is characterized in that the amplifying and/or impedance characteristics of said termination circuit and/or the termination reference voltage, and/or the termination averaging impedance are deviant to generate a current in said output voltage terminal of said outer amplifying circuit of said array, that is corrected for the finite size of the array.
In other words, a termination circuit deviates in at least one of its constituting components from the amplifying circuits of the array. That is, the amplifying and/or impedance characteristics of said termination circuit deviate from the generally uniform amplifying and/or impedance characteristics of the amplifying circuits of the array; and/or a voltage difference between said termination reference voltage and a reference voltage received by an outer amplifying circuit deviates from a voltage difference between reference voltages of two adjacent amplifying circuits in the array; and/or a termination averaging impedance deviates from averaging impedances used in said array.
Throughout the remainder of the text, the term xe2x80x9cdeviantxe2x80x9d or xe2x80x9cdeviatesxe2x80x9d is used in contrast to the term xe2x80x9cgenerally uniformxe2x80x9d in the above referenced context. The AD-converter according to the invention is especially beneficial since it offers a termination circuit with a minimum number of components, similar in design as the amplifying circuits of said array, thus eliminating the need for complicated, slow and/or power-absorbing extra circuits, while at the same time boundary effects may be eliminated rigorously, in contrast to the termination networks of the prior art, which merely seek to achieve a less-rigorous compensation for said boundary effects.
In a preferred embodiment, the amplifying and/or impedance characteristics of said termination circuit and/or the termination reference voltage, and/or the termination averaging impedance of said termination network deviate so that currents through each averaging impedance between adjacent output voltage terminals of said array are generally equal in magnitude. In this way, according to the invention, a termination of the AD-converter may be provided, wherein linearity of the array is optimally restored. The zero crossing of the termination amplifier circuit is shifted strongly, and therefore not usable, in favor of the recovered position of the zero crossing of amplifiers in the array. Such a redefinition of the edge termination circuit not only recovers the linearity at the edge, but also prevents propagation of an imbalance between amplifiers.
In one embodiment, the invention is characterized in that a voltage difference between said termination reference voltage and a reference voltage received by an outer amplifying circuit is larger as compared to the generally uniform spacing between each reference voltage received by said amplifier circuits, wherein the amplifying and impedance characteristics of said termination circuit and the termination averaging impedance are not deviant.
Specifically, in a preferred embodiment according to the invention, in an array comprising amplifier circuits having an output impedance of R1 and coupled by averaging impedances having a value of R2, the voltage difference is preferably a factor       R1    +    R2    R2
larger compared to said uniform spacing. In this preferred embodiment an output terminal may also be coupled to the termination circuit, since the voltage difference over the averaging resistor is generally equal to the differences of the averaging resistors of the rest of the amplifying circuits. This modification has as a special advantage that no dummy circuit whatsoever needs to be active in order to restore linearity properties.
In another preferred embodiment, the termination circuit has a zero output impedance wherein the amplifying characteristics of said termination circuit, the termination reference voltage and the termination averaging impedance are not deviant.
In yet another preferred embodiment, in an array comprising amplifier circuits having an output impedance of R1 and coupled by averaging impedances having a value of R2, the termination averaging impedance equals R2xe2x88x92R1, wherein the amplifying and impedance characteristics of said termination circuit and the termination reference voltage are not deviant. Again, since the averaging impedance is left unaltered, an output terminal may also be coupled to the termination circuit.
In still another embodiment wherein R1 greater than R2, in an array comprising amplifier circuits having an output impedance of R1 and coupled by averaging impedances having a value of R2, the termination averaging impedance may be created by an active circuit, wherein the amplifying and impedance characteristics of said termination circuit and the termination reference voltage are not deviant. By such an active circuit, an effective negative impedance may be created in order to achieve the termination according to the invention.
Specifically, in the case wherein R1 greater than R2, the termination network may comprise a plurality of termination circuits, wherein at least two output voltage terminals of two adjacent termination circuits are short-circuited. Therefore, according to an aspect of the invention, by short-circuiting the outer output voltage terminals, linearity of the inner output voltage terminals is restored.
More specifically, the termination network comprises two termination circuits comprising output voltage terminals which are short circuited, the amplifying circuits further receiving an equally spaced referenced voltage and being connected to the input voltage, wherein the output voltage terminals of said amplifying circuits are coupled by a termination averaging impedance to an output voltage terminal of an outer amplifying circuit of said array, the termination averaging impedance equaling                     3        2            ⁢              xe2x80x83            ⁢      R2        -                  1        2            ⁢              xe2x80x83            ⁢      R1        ,
wherein the amplifying and impedance characteristics of said termination circuit and the termination reference voltages are not deviant.
The invention further relates to a method of designing an AD-converter, wherein the amplifying and impedance characteristics of said termination circuit and the termination reference voltage are generally uniform in magnitude, in an array comprising amplifier circuits having an output impedance of R1 and coupled by averaging impedances having a value of R2, wherein R1 greater than R2, the method comprising the steps of: short-circuiting two output voltage terminals of two adjacent termination circuits; and the method further comprising repeating the steps of: calculating a magnitude of a first termination impedance which couples an output voltage terminal of said termination circuit to a first output voltage terminal of an amplifying circuit adjacent to said termination circuit, so that currents through each averaging impedance between adjacent output voltage terminals of said array are generally equal in magnitude; and if said magnitude is negative, short circuiting said output voltage terminal of said termination circuit to said first output voltage terminal and calculating a magnitude of a next termination impedance which couples an output voltage terminal of said termination circuit to a next output voltage terminal of an amplifying circuit adjacent to said termination circuit.
According to the method of the invention it becomes apparent, that, even if a first step calculation indicates a design where unphysical properties of the termination averaging impedance are required, such a requirement can be met by xe2x80x9cofferingxe2x80x9d an outer amplifying circuit of the array, which thereby becomes a part of the termination network; and recalculate the desired termination impedances of the next adjacent termination circuit. In this way, the required termination averaging impedance will become more positive, and after repeating the above mentioned steps, a positive dimensioned termination averaging impedance can be calculated for correcting the array according to the invention.